Add warnings for more scenarios

[swissspidy]

Previously: #33.

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The WordPress Coding Standards project has a few sniffs that we could borrow from to improve the warnings created by the make-pot command.

These sniffs currently do:

• Check for the presence of a translators comment if one of the text strings contains a placeholder.
• Check for inconsistencies between single and plural arguments.
• Check for multiple unordered placeholders.
• Check that the translatable string itself is not empty

Source: https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/blob/eccb1f721f95fdcf5c2e383340aa4b094c0317ad/WordPress/Sniffs/WP/I18nSniff.php#L36-L59

Let's determine whether it makes sense to port some of these over to WP-CLI.

[swissspidy]

Fixed by #64.